Wireless digital communications systems are poised to offer a cost-effective alternative to cable and DSL data services. So called “WiMAX” technology, based on the IEEE 802.16e air interface standard is a promising framework for broadband wireless applications. It has the potential to enable full internet and digital voice services for both fixed and mobile users.
The physical layer architecture for IEEE 802.16e OFDMA systems is based on orthogonal frequency-division multiplexing (OFDM) modulation. Since OFDM divides the total bandwidth into multiple narrowband sub-bands, the effects of frequency selective fading are reduced. The OFDM system allows for a simple receiver structure while maintaining high link quality. The technology also employs adaptive modulation and coding in both the downlink and the uplink to deal with variations in link quality. This enables WiMAX to offer multiple date rates at the physical layer which can be adapted dynamically based on the integrity of the air link.
Multiple users share the total system bandwidth by multiplexing their data in both time and frequency. In an adaptive OFDM system, spectral efficiency can be improved by allocating time-frequency resources based on throughput requirements, quality of service constraints and the channel qualities of each user. A scheduler, which optimizes resource allocation for multiple active users, becomes a key element in such a solution. In present code-division multiple access (CDMA) systems, spectral efficiency decreases with an increasing number of active users because of intra-cell interference due to imperfect orthogonality of the downlinks. In an adaptive OFDM system, where orthogonal time-frequency resources are given to the user who can utilize them best, the spectral efficiency instead increases with the number of active users. This effect is known as multiuser diversity.
To take greatest advantage of multiuser diversity, channel state information must be available at the transmitter. When perfect channel information exists, i.e. knowledge of the signal to interference and noise ratio (SINR), for every possible time-frequency bin, an optimal assignment of spectral resources can be accomplished. Of course, in reality channel state information is never perfectly known.
Conversely when poor or no channel information is available, the best allocation of spectral resources may be to distribute time-frequency bins to users randomly across the entire system bandwidth. This mode of operation is known as frequency diversity.
Particularly in the case of mobile users, where channel state information is not only imperfect, but also constantly changing, neither multiuser diversity nor frequency diversity represents an optimal solution. Multiuser diversity may not work well in a changing mobile communications environment where the quality of channel state information is variable. On the other hand, frequency diversity ignores channel state information that may be useful even though it is imperfect.
What is needed is a mode that combines the best features of multiuser diversity and frequency diversity modes. Such a solution would enable WiMAX and other OFDMA digital communications systems to achieve better net spectral efficiency.